Now I can use scale factor 4 but any larger than that will produce the same error in ACDSee. Although now the files are larger than 0 KB but ACDSee still says "The source data format is recognized." I have also tried IrfanView, photoshop, Paint and others but with no luck. Blockland does take the screenshot successfully.
Now, here's a proper comparison for the people who still haven't realized what megashot does. All the screenshots were taken with Blockland resolution 1280x720. Below are the screenshot comparisons that I have resized down to 800x600.
Original resolution 1280x720.

Original resolution 2048x1536. Megashot scale factor 2.

Original resolution 4096x3072. Megashot scale factor 4.

The difference is quite hard to see with naked eye. But let me make it so obvious that anyone can understand it. Here I have taken the original screenshots, not resized down to 800x600, and simply resized the
canvas of the image down to 300x300 pixels. That means the rest of the image is cropped off and a 300x300 pixel area of the center of the image is left.
Original resolution 1280x720.

Original resolution 2048x1536. Megashot scale factor 2.

Original resolution 4096x3072. Megashot scale factor 4.

As you can see, the tree near the center of the image gets bigger the larger the Megashot scale factor is, because the screenshot itself that you take with Megashot is bigger.
Here are the original screenshots.
1280x720.
http://blockland.dataorb.net/files/screenshots/Blockland_00207.png2048x1536
http://blockland.dataorb.net/files/screenshots/Blockland_00208.png4096x3072
http://blockland.dataorb.net/files/screenshots/Blockland_00209.png